1. Field of the Invention
This invention relates to oxide magnetic materials, making method, antenna coils, and inductance elements for preparing the same.
2. Prior Art
Oxide magnetic materials are often used to form magnetic cores which find use in antenna coils and inductance elements built in pagers (or beepers) and other electronic equipment adapted to operate in high-frequency bands of 200 to 500 MHz.
As disclosed in JP-A 169005/1988, such oxide magnetic materials are used as magnetic cores in inductance elements such as band filters in electronic equipment adapted to operate in high-frequency bands of about 200 MHz or higher.
Loop antennas with air-core coils are often used in pagers. The loop antennas have the advantage of high Q, but are difficult to adjust inductance and other properties which are, in turn, susceptible to fluctuation by external forces such as vibration. To increase the inductance, the number of turns of winding must be increased, resulting in a larger size of coil as a whole.
In contrast, coils with cores made of an oxide magnetic material as disclosed in the above-referred JP-A 169005/1988 have the advantages of easy adjustment of inductance and size reduction, but the disadvantage of low Q.
An improvement in Q of oxide magnetic material is disclosed in JP-A 339913/1996. This oxide magnetic material is successful in achieving high Q, but has an undesirably high temperature coefficient of inductance. Typical examples are oxide magnetic materials consisting essentially of 23 mol % of Fe.sub.2 O.sub.3, 9 mol % of CuO and 68 mol % of NiO as main ingredients and containing PbO, SiO.sub.2 and CoO as additives. Table 1 shows the initial permeability (.mu.i), the Q value at 280 MHz and the temperature coefficient of inductance (.alpha..mu.r) at 100 kHz of these materials.
TABLE 1 ______________________________________ PbO SiO.sub.2 CoO .alpha..mu.r (wt %) (wt %) (wt %) .mu.i O (ppm/.degree. C.) ______________________________________ 3.0 1.7 2.0 3 300 380 3.0 0.7 1.0 3 250 327 ______________________________________
As seen from Table 1, these materials are improved in Q, but have a high temperature coefficient of inductance which is undesirable for use as high-frequency inductors such as antenna coils.
As described above, loop antennas are conventionally used in pagers and other electronic equipment. Despite the advantage of high Q, the loop antennas are generally of large size as a whole. Coils using magnetic cores of the oxide magnetic material of JP-A 339913/1996 can be reduced in size because of high inductance and have high Q, but suffer from a high temperature coefficient of inductance. Since a high temperature coefficient of inductance means that inductance largely varies with temperature changes, antenna coils made of such oxide magnetic material become unstable in tuning frequency and sensitivity.